The present embodiments relate to an operating method for a magnetic resonance system.
Magnetic resonance systems use real-time controllers in a transmit chain in order to keep a high frequency amplitude of a transmit pulse constant. Controllers of this type may use a signal present at a radio frequency power amplifier (RFPA) output as a control variable. The signal is down-mixed to the base band. A signal is modified as an actuating variable. The signal is likewise in the base band range.
The bandwidth of controllers of this type may be restricted. Bandwidths may lie at approximately 100 kHz. This is sufficient to output amplitude-modulated high frequency (HF) pulses (e.g., with bandwidths in the range of a few kHz) in a distortion-free manner. Frequency offsets in the range of up to 250 kHz are, however, provided for the off-center excitation. Frequency shifts of this type are realized by an oscillator. The oscillator outputs a correspondingly varied frequency signal. The frequency offset is therefore transparent for the control process.
The modulation also takes place in the base band in the case of frequency-modulated pulses. If the bandwidth of the HF pulse resulting herefrom is larger than the controller bandwidth, the pulses are either no longer output in a distortion-free manner or the controller is shut down. One example of a pulse of this type is the hyperbolic secant (HS) pulse. A further example is the frequency offset corrected inversion (FOCI) pulse.